Phase trimmer for strip transmission line



'Dec. 12, 1961 R. A. JORDAN 3,013,227

PHASE TRIMMER FOR STRIP TRANSMISSION LINE Filed Oct. 5, 1960 l IIHIIHHIIIHHII m: w m

MN" im I 12 Z 3 I o E 2 2 o INVENTOR ROBERT A. JORDAN Fig BY fla ATTORNEY 3,013,227 PHASE TER Fglgglfl TRANSMISSION Robert A. Jordan, Essex, Mass, assignor to Sylvania Electric Products Inc, a corporation of Delaware Filed Get. 3, 1960,5er. No. 60,208 5 Claims. (Cl. 333-31) This invention relates generally to microwave apparatus and is more particularly concerned with a phase trimming device for providing terminal phase adjustment for electrical energy transmitted through strip transmission line networks.

In certain microwave systems, for example in phased array radar detection systems, it is frequently necessary to divide a signal into a multiplicity of output signals and distribute the resulting wave fronts with phase uniformity among a number of output loads. Normally, this distribution of signals is performed by a power divider of the multiple output strip-line type, a representative example of which is shown in FIG. 1. -A common input signal being applied to the divider, the electrical specification of this device is normally defined in terms of its output characteristics, the specifications including: the equality of the power division; the amount of passive attenuation to the transmitted signal; the degree of isolation between the output channels; and the phase differential between the multiple output signals. The latter characteristic of the divider is the primary concern of the present invention.

In a strip transmission line power divider, and indeed in most power dividers, the electrical phase shift experienced by any output signal is a function of the physical length of the conducting path for that signal between the input and output terminals. Thus, it is obvious that to satisfy the requirements of uniform output phase care must be taken in the construction of the multiple paths of the divider to insure that they are all of the same length. In applications where minor phase differentials betweeen the output signals are tolerable, this requirement ordinarily introduces no major fabrication problems. In the strip transmission line case, the pattern of the inner conductor of the divider is carefully planned with a tape or ruler such that the length of the inner strip conductor between the common input terminal and each output connector is measurably constant. To this end, the strip line contour of an even-numbered output divider is usually laid out in a pattern which is symmetrical about the input terminal as shown in FIG. 1. However, in applications where system specifications demand a more exacting tolerance in the phase of the output'signals, for example of the order of a few tenths of a degree, customary layout procedures have been found to be inaccurate and unacceptable. The wavelength of a microwave signal being very small, incremental errors in the length of adjacent signal channels introduced in the design and fabrication of the divider tend to accumulate and result in significant phase differences between the several output signals. Indeed, even in the best planned and executed circuit layout, such accumulation of errors may be expected to introduce at least a one degree phase error at 400 megacycles per second, an error which cannot be tolerated in many systems. Heretofore attempts to compensate for such errors have taken the trial and error approach which required each strip-line channel to be repeatedly reconstructed or realigned until the output signal satisfied the specifications with respect to phase. This approach is obviously costly and time consuming and generally unsatisfactory. Another approach has been to incorporate a phase shifting device in each signal channel just ahead of its output connector. Such devices included a rotatable control located in the upper ground plane of the strip transmission line system, thus permitting the adjustment of the phase of each of the output signals. This solution has been found to be operationally acceptable, but it is unduly expensive and becomes impractical in situations where space limitations require stacking of a number of dividers.

With an appreciation of the foregoing limitations of prior art power divider construction, applicant has as the primary object of this invention to provide an improved phase-trimming device for strip transmission line networks.

Another object of the invention is to provide a phasetrimming device for strip transmission line networks which introduces negligible attenuation and discontinuity to the signal channel of the network in which it is incorporated.

Another object of the invention is to provide a phasetrimmer useful in a power divider system having the foregoing features which may be readily adjusted even when a number of dividers are stacked together.

Still another object of the invention is to provide a phase-trimming device which is of simple and inexpensive mechanical construction.

Briefly stated, the phase-trimmer according to the invention employs the principle of inserting a mismatched section of transmission line of adjustable length in series with a signal channel just ahead of its termination. In a specific embodiment, the phase-trimmer consists of a coaxial connector having a rotatable center pin which is slotted on the external end to accommodate a screwdriver. The inner end of the center pin is threaded into a sleeve which serves as one conductor of a slab-line transmission line whose characteristic impedance differs from the characteristic impedance of the conducting mediums I immediately preceding and succeeding the device.

The slab-line sleeve is movable along the axis of the pin in response to rotation of the center pin of the connector.

This sleeve is aligned with the final section of the strip 1 transmission line system and coupled thereto bylmeans of a tapered conductive tab which 'slides along thelsu'rface of the inner conductor of the strip transmission line as the center pin is rotated. The sliding tab permits the mismatched section of slab-line between the final stripline section and the center pin of the connector to be varied in length without introducing an actual change in the overall length of the signal channel. The phase of the output signal, being functionally related to the length of the mismatched section of line, may therefore be adjusted simply by rotating the center pin of the connector.

Other objects, features, and advantages of the invention and a better understanding of its construction and operation will be apparent from the following detailed description, taken in conjunction with the accompanying draw ings, in which:

FIG. 1 is a perspective view of an multiple output strip transmission line power divider of the type referred to above and in which the invention is especially useful, the upper ground plane of the strip-line being removed to illustrate the divider network;

FIG. 2 is a perspective view illustrating the application of the invention in a typical strip transmission line circuit, the upper ground plane being removed to show the inner conductor and the coupling thereto of the phase trimmer;

FIG. 3 is a fragmentary elevation cross-section view taken along the line 33 of FIG. 2; and

FIG. 4 is the equivalent circuit of the phase-trimmer structure of FIG. 3, useful in explaining its operation.

Referring briefly to FIG. 1, the present invention has particular applicability in a strip transmission line power divider which functions to divide an input signal, applied at termnal it), into a plurality of equi-amplitude, equiphase signals at output couplers 12 through 26... To this end, the inner conductor 28 of a strip transmission line having a lower ground plane 30 (and an upper ground plane which has been removed in the illustration to show the inner conductor), is laid out in a pattern such that the physical length of the conductor between the input terminal and each of the output couplers is equal. For example, the physical length between the input 10 and terminal 14 is the same (insofar as can be discerned) as the length between input terminal 19 and output coupler 2t). It is extremely difiicult, however, in the fabrication of a complex system such as this to insure that the eight paths are absolutely identical so as to give equi-phase output signals.

In accordance with this invention, a phase-trimming device as shown in FIGS. 2 and 3 is incorporated in each of the transmission paths just ahead of the output coupler. As shown in FIG. 2, the strip transmission line includes a lower ground plane 30 of conductive material on which is supported a layer of dielectric material 32 on which, in turn, is supported the inner conductor 23 of the transmission line, usually in the form of a thin metal foil. The transmission line is completed by a second slab of dielectric material placed over the conductor 28 and an upper ground plane, the latter elements being removed in FIG. 2 to show the construction of the phase trimmer. The ground planes and dielectric slabs are sandwiched together by suitable securing means such as the bolts 34. Connection is made to the line by a coaxial connector 12, the outer conductor 12a of which is conductively secured to both of the ground planes by screws 36 threaded into metallic plate 38, which, in turn, is secured to the edges of the ground planes. The inner conductor 23 of the strip transmission line terminates short of the edge to which the connector is secured, connection thereof to the inner conductor 12b of the connector being made through the phase trimmer now to be described.

In accordance with the invention, the center conductor 12b of the coaxial connector 12 is rotatable relative to the outer conductor 12:: and may be provided by removing the splines on the center conductor of a commercially available coaxial connector in the region where it passes through the dielectric insulating bushing 12c. The shoulder 12c and C-ring 12d normally found in commercial connectors are retained to prevent endwise motion of the center conductor as it is rotated. Finally, the outer end of the inner conductor is slotted as shown at 12f to accommodate a screwdriver or similar tool for rotating the inner conductor.

To the inner end of the rotatable center pin 12b is secured a threaded conductive pin 40 which is threaded into an internally threaded sleeve 42. The inner end of sleeve 42 is, in turn, secured to a fiat electrically conductive tab having a width corresponding to that of inner conductor 28 of the strip-line and tapered to a width corresponding to the diameter of sleeve 42 to provide a gradual impedance transition between the tab and the sleeve. The dielectric slab 32, and the corresponding upper slab, are formed with semi-cylindrical grooves to accommodate the threaded pin 40 and sleeve 42 and to permit motion of the sleeve 42 in a direction parallel to its axis upon rotation of center conductor 12b and sliding movement of tab 44 on conductor 28, the latter elements being maintained in firm contact by the upper slab of dielectric material.

In effect, the described structure constitutes a transition from a strip transmission line to a short section of slab transmission line, consisting of a portion of sleeve 42 and the upper and lower ground planes, and thence to a coaxial transmission line consisting of the threaded pin and the inner and outer conductors of the coaxial connector 12. More specifically, a signal propagated along conductor 28 sees as transmission line loads the thin metallic tab 44, which may be assumed to have the same characteristic impedance as that of the strip transmission line, a section of slab-line of length I (that portion of sleeve 42 which does not contain the pin 40) having a diiferent characteristic impedance, and the pin 40 and the center conductor 12b to which it is secured, the latter structure having an impedance equal to that of the transmission line assumed to be connected to the coaxial connector (not shown in FIG. 3). Assuming that the strip transmission line is designed to have the same characteristic impedance Z as the output coaxial transmission line, the slabline of length I therefore presents the only mismatch in the transmission line, the impedance of this short section being designated for purposes of the following discussion as Z As the center pin is rotated it is apparent that the degree of insertion into sleeve 42 varies, or conversely, that the length of the slab-line section is altered. Inasmuch as there is continuous contact between the tab. 44 and the conductor 23, the effective length of the strip transmission line section does not change. The net result of the adjustment is a proportional change in the phase of the output signal which is a function of the length l of the series mismatch introduced by the slab-line section as shown by the following derivation of the equivalent circuit of FIG. 4.

As mentioned previously. the equivalent circuit of the device can be considered to consist of a section of transmission line of variable length l and characteristics impedance Z bounded by two sections of transmission line each of characteristic impedance Z Accordingly, the relationship between the phase of an output signal and the length l of the mismatch in series with the signal path may be derived by analyzing the three-section transmission line of FIG. 4. Applying the well known transmission line equations to the intermediary transmission line in the figure;

E1=E2 COB 6+IgjZ0 Bill 0 I j sin 0 1) +1 cost? Rearranging the equations into the popular ABCD matrix for a lossless section of transmission line, we have:

AB cos GjZ sin 0 CD (Eq. 2)

jsint) Z0 cos 0 The propagation factor e" of the line is defined as:

or in terms of the matrix e "=(A+B)(C-l-D)=AC+BC+AD+BD (Eq. 4)

Terminating the phase-shifting section line of characteristic impedance Z with the characteristic impedance Z of the output transmission line we obtain for the propagation function:

sin226 sin Z In Equation 5,

0=l2lf\/e (Eqwhere l is length in centimeters, f is frequency in kilomegacycles, and e, is the relative dielectric constant of the transmitting media.

at an angle of 1 .1 W an 2 tan 2 2Z0 g 0 in Equation 6 represents the phase shift experienced by a signal of frequency traveling through a line of length I if terminated by a matched load. 5 in the expression preceding Equation 7, on the other hand, represents the phase shift experienced by a signal transmitted through a mismatched line of the same physical length and terminated by a matched load. 'If the expression of Equation 6 be subtracted from the expression of Equation 7, it follows that the net result must be equal to the change in phase, 13, resulting from the mismatched condition; or

In terms of the parameters of the present invention, it will be seen from Equation 8 that At, the net phase shift, is a function of "2 the ratio of the mismatched impedances at the junction of the slab line and the center pin of the connector, the frequency f of the transmitted signal, the relative dielectric constant e of the transmitting media, and finally the length l of the series mismatched section of slab line. The phase trimming structure of present invention converts the net phase shift, A in Equation 8 into a readily controllable variable by making the parameter l, the length of the mismatched line, adjustable at an accessible point exteriorly of strip transmission line.

Inasmuch as the propagation constant in Equation 7 is a complex function, it would be expected that the mismatch would introduce some attenuation and standing waves. In practice, however, these undesirable characteristics have been found to be negligbile. For example, in a phase trimmer constructed for use in the 400 megacycle frequency range, and consisting of a 50-ohm coaxial connector terminated in a matched load, and a one-half inch long section of slab-line connected to one output of a stripline divider network and designed to have an impedance of 75 ohms, tests have shown that the device contributed a voltage standing wave ratio of less than 1.05, and series attenuation which was practically immeasurable but calculated to be in the order of 0.5 decibel, while providing a phase variation in the output signal up to 0.6 degree.

From the foregoing description it is seen that applicant has provided a phase-trimming device of simple construction which is very effective in adjusting the effective electrical length of a strip transmission line. Adjustment is readily made with simple tools externally of the transmission line system, with suitable increments in phase shift obtainable with relatively small linear movement of the sleeve and tab. Moreover, the phase-trimming device does not introduce significant attenuation nor does it appreciably aifect the voltage standing wave ratio of the transmission line. Thus, it finds particular application in precision type microwave power dividers for compensating for some of the cumulative inaccuracies which are unavoidably present in complex circuit layouts. It is to be understood, however, that the invention is not limited to power dividers, but may be employed in any strip transmission line circuit where it may be necessary precisely to adjust the phase of a signal.

What is claimed is:

1. An electrical phase-trimming device comprising, a

coaxial connector having an outer conductor and an inner conductor rotatable relative thereto, a section of transmission line having a characteristic impedance which differs from the characteristic impedance of said coaxial connector inserted in series between said inner conductor and a signal transmission medium for which the device is a termination, a portion of said inner conductor being threadably joined to said section of transmission line for moving said section along the axis of said inner conductor in response to rotation of said inner conductor to introduce a variation in the length of said transmission line section.

2. Electrical apparatus comprising, in combination, a strip transmission line including a flat inner conductor, a coaxial connector for said transmission line having an outer conductor and an inner conductor rotatable relative thereto, an internally threaded sleeve colinear with said inner conductor slidably and conductively joined at one end to said flat inner conductor, and a conductive pin secured to the inner conductor of said coaxial connector threadably engaging said sleeve and operative upon rotation thereof to adjust the amount of its insertion into said sleeve.

3. A phase-trimming terminating device for a strip transmission line having a fiat inner conductor, said device comprising, a coaxial connector having an outer conductor and an inner conductor rotatable relative thereto, said inner conductor having a threaded extension disposed within said strip transmission line, and an internally threaded conductive sleeve engaged at one end by said threaded extension and slidably joined at the other end to said flat inner conductor, the portion of said sleeve not containing said extension constituting one conductor of a slab transmission line, the length of which is adjustable in response to rotation of said inner conductor.

4. Electrical apparatus comprising, in combination a strip transmission line including a pair of ground planes and a flat intermediate conductor, a coaxial connector having an outer conductor and an inner conductor rotatable relative to the outer conductor, means conductively securing the outer conductor of said connector to the ground plane of said strip transmission line, and a section of slab transmission line connected in series between the fiat conductor of said strip transmission line and the inner conductor of said connector, and means operative in response to rotation of said inner conductor for adjusting the length of said slab transmisison line.

5. A phase-trimming device for a strip transmission line including a pair of parallel ground planes spaced apart by a dielectric and an intermediate flat conductor, said device comprising, in combination, a coaxial connector having an outer conductor and an inner conductor rotatable relative thereto, means for conductively securing said outer conductor to said ground planes, said inner conductor having a threaded colinear extension disposed within the dielectric of said strip transmission line, an internally threaded sleeve engaged at one end by said threaded extension, a flat conductive tab secured to the other end of said sleeve and slidably positioned on the fiat conductor of said strip transmission line, said sleeve with said ground planes constituting a slab transmission line of a length depending upon the amount of insertion of said extension and having a characteristic impedance differing from that of said strip transmission line and said coaxial connector, the length of said slab transmission line being adjustable in response to rotation of said inner conductor.

References Cited in the file of this patent UNITED STATES PATENTS 2,644,140 Please June 30, 1953 2,786,184 Alford Mar. 19, 1957 

